The engine used by Chromium (Blink) is easier to use for programmers, since it’s designed to be “embeddable” from the start when it was still known as KHTML. Engine used by Firefox (Gecko) is only kind of embeddable as the Mozilla developers haven’t paid much attention for that a long time, which makes it more difficult to use in your own browser that you develop.
The engine used by Chromium (Blink) is easier to use for programmers, since it’s designed to be “embeddable” from the start when it was still known as KHTML. Engine used by Firefox (Gecko) is only kind of embeddable as the Mozilla developers haven’t paid much attention for that a long time, which makes it more difficult to use in your own browser that you develop.